EPNM 2012
Metallurgical Considerations in
Hot Metalworking Bi-Metal
Materials
John Banker
Vice President Customers & Technology
Dynamic Materials Corporation
Boulder, CO, USA
EPNM 2012
Introduction
 Explosion Welding is a proven, robust technology
for manufacture of flat clad plates and concentric
cylinders
 Fabrication of these clad components into
industrial equipment often requires high
temperature metalworking operations
 Controlling these procedures to simultaneously
assure metallurgical quality of the cladding layer,
base layer and interface in the fabricated
equipment can be challenging
EPNM 2012
Hot Cylinder Forming
 Commonly used for
clad steel >100mm
thick
 Typically performed at
approximately650 C to 900 C
Dependent upon steel
composition and thickness
EPNM 2012
Hot Head Forming
 Commonly used for
clad steel >25 mm tk
 Typically performed at
approximately500C to 1100 C
Dependent upon clad
combination, steel
type, and thickness
EPNM 2012
Hot Plate Rolling
“Bang and Roll”
 Typically used to produce
clad plates <20 mm tk
from thicker clad slabs
 Performed at
approximately750 C to 1150 C Dependent
upon clad combination, steel
type, and thickness
EPNM 2012
Base Metal Metallurgical Concerns
 Steels are the dominant clad base metal
 Steel is selected for mechanical strength and
toughness
 Base metal must meet Specification
Requirements after hot working, heat
treating, and all fabrication work (UTS, YS,
Impacts, etc)
EPNM 2012
Typical Steel Hot Working Ranges
Austenitic Hot
Working Range
Figure 4. Iron-Carbon Phase Diagram
Ferritic Hot
Working Range
Typical Structural Steel Carbon Level
EPNM 2012
Temperature (°C)
Typical Steel Heat Treatments to
Achieve Required Properties
900
Austentize
650
Normalize
(still air
cooling)
Water Quench
Cooling +
Temper
20
Time (min)
EPNM 2012
Cladding Metal Metallurgical Concerns
 Corrosion Resistant Alloy (CRA) clad
 Cladding metal is selected for specific corrosion
resistance
 Cladding metal is rarely considered in design
strength calculations
 High temperature operations must be controlled to
assure corrosion performance
 Key Factors: temperature, time at temperature, heating
and cooling rates
 Time-Temperature-Sensitization Curves show
relationships of Corrosion Properties vs Heat Treatments
EPNM 2012
CRA Groups from Hot Working
Perspective
 Stainless Steels:
 Basic austenitic stainless steels- 304, 316, 321, 347,
317
 Super austenitic stainless steels- >= 5% Mo
 Duplex stainless steels
 Nickel Alloys
 Reactive metals- Ti & Zr
 Refractory metals – Ta & Nb
EPNM 2012
Secondary Phases of Concern in
Stainless Steels and Nickel Alloys
Phase
Composition
Temperature Range
Structure
(Cr,Fe,Mo)23C6
600 – 950° C
Cubic
(Cr,Fe,Mo,Cb)6C
600 – 950° C
Cubic
(Cr,Fe,Mo)7C6
950 - 1050° C
Orthorhombic
Nitrides
Sigma σ
(Cr,Fe)2N
(Cr,Fe,Mo,Ni)
650 – 950° C
550 – 1050° C
Hexagonal
Tetragonal
Laves η
(FeCr)2(Mo,Nb,Ti,Si)
550 – 900° C
Hexagonal
Chi χ
Fe36Cr12Mo10
600 – 900° C
Cubic
Carbides
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Austenitic Stainless Steels
Carbide Formation Major Concern
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Increased Alloying – Other Phases
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Duplex Stainless Steels
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Nickel Alloys
 Some Alloys Very Slow or No High
Temperature Sensitization
 Nickel 200: Unalloyed Ni
 Alloy 400: 60 Ni – 40 Cu
 Alloy 625: 61 Ni + 22 Cr + 4 Fe + 3.6 Cb + 9 Mo
 Alloy 825: 42 Ni – 22 Cr – 32 Fe + 2.2 Cu + 3 Mo
 Others Complex secondary phase
development- Example Alloy C Family
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Nickel Alloy C Family Ni-Cr-Mo
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Nickel Alloy C Family Ni-Cr-Mo (alt)
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Reactive & Refractory Metals
Commonly used Reactive and
Refractory metals exhibit no significant
phase changes that affect corrosion
performance
Titanium alloys Grades 1, 2, 16, 17
Zr alloys 700 & 702
Tantalum and Ta-2.5W
EPNM 2012
Effects of Hot Working on Clad
Interface Properties
Stainless Steels & Nickel Alloys
Diffusion w/in +/- 0.5mm of interface
No continuous brittle intermetallics formed
Slight decrease in shear strength due to
recovery of Cold Work at interface
Interface retains toughness
Extremely difficult to disbond
EPNM 2012
Reactive Metal Clad Interface
Concerns during Hot Working
 Ti-Fe, Zr-Fe form
several brittle
intermetallic
compounds
 Exposure to Elevated
Temperatures can
Degrade Clad Interface
Properties
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Shear Strength of Titanium-Steel
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Larson-Miller Parameter
Titanium – Steel Clad
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Shear Strength of Zirconium-Steel
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Reactive Metal Clad Steel
Hot Working Considerations
 Optimum temperature range for Heat Teating
and Hot Working Reactive Metal Clad is
between 550oC – 700oC
 Avoid unacceptable degradation of interface
properties
 Reduction in base metal yield strength at forming
temperature
 Below steel lower critical temperature
• Minimize changes to base metal structure and
mechanical properties
EPNM 2012
Titanium-Steel & Zirconium-Steel Formed Heads
80
75
Total Thickness (mm)
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Formed Head Diameter (mm)
Titanium Clad - Segmental
Titanium Clad
Zirconium Clad - Segmental
Zirconium Clad
5000
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Typical Segmental Ti Clad Head
9 m Diameter x (80 + 3) thick
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Hot Working Refractory Alloy Clad
Tantalum
Readily oxidizes in air above 300C
Cold working is typical
If hot working is mandatory, tantalum
must be protected from air
Some success with encapsulation in
steel
EPNM 2012
Bang & Roll
Stainless Steels & Nickel Alloys
 Typical rolling at 1100 C to 900 C
 Single slab rolling for clad thickness <10% of total
thickness
 Pack rolling for greater clad %
 Reliable clad thickness uniformity
 Reliable product yields
 Some cladding alloys not possible to achieve both
cladding metal and base metal properties
Examples: Duplex, Some C-family alloys
EPNM 2012
Bang & Roll
Titanium – Steel Clad
 Considerable tonnage of titanium clad has been produced
by single slab Bang & Roll
 Slabs up to 150mm thick, rolled product down to 5mm
thick
 Good thickness uniformity and rolling control up to 15%
clad ratio
 Difficult to achieve both good bond strength AND base
metal of Pressure Vessel Quality
 Test work by Hardwick indicates that a Nb interlayer may
allow higher temperature rolling, possibly reducing the
steel quality issues
EPNM 2012
Conclusions
 Explosion Clad plates can be formed and
fabricated into reliable industrial equipment
by Hot Metalworking
 With Proper Selection of Alloys & Procedures,
no compromise in
 Cladding Metal Corrosion Properties
 Base Metal Mechanical Properties
 Clad bond quality
 With the Wrong Procedures, it is easy to
make expensive trash quickly